There are many applications where it is desired to have a low drag method of delaying or preventing flow separation between a flowing fluid and a flow-control surface in regions where, due to contours of the flow-control surface, the fluid's boundary layer is subjected to adverse pressure gradients normally sufficient to cause flow separation.
The purpose of such boundary layer control is to delay or prevent flow separation. When a viscous fluid boundary layer flows from a region of low static pressure to a region of high static pressure, as when passing over a wing toward the trailing edge, it is said to be in a region of adverse pressure gradient. This results in forces tending to retard the boundary layer which can become strong enough to arrest and reverse the flow. This in turn causes the fluid to separate from the flow-control surface and no longer be influenced predictably by the downstream contour. The result is increased drag due to the large cross-sectional area of separated flow in the wake of the flow control surface. In the case of a stalled wing, degraded lift is accompanied by a rise in drag.
One useful but relatively high drag method heretofore used is to generate streamwise vortices for the purpose of mixing free-stream flow with the boundary layer to delay or totally prevent flow separation. Generally, there are two categories of vortex generators. The first category covers those devices that protrude well into and usually somewhat above the 99% boundary layer height. Included in this category are conventional vane-type vortex generators as well as prior art ramp-type generators of varying planform shapes. Unfortunately, these devices achieve boundary layer control only at the penalty of considerable drag. An example of the ramp-type vortex generator is shown in STEPHENS, U.S. Pat. No. 2,800,291. STEPHENS teaches the use of ramp-type generators positioned in rows at right angles to the flow with the generators extending back to some intermediate position on the flow-control surface.
The second and less familiar category of vortex generators uses the "Taylor-Goetler instability" to generate streamwise vortices within the boundary layer when a fluid is caused to flow over a concave surface. This alternative is more subtle, but the vortices are still left to travel downstream on the primary flow-control surface, causing turbulent mixing between the boundary layer and the free-stream and resulting in an unwanted drag increment. Devices of this category are disclosed by KUETHE in U.S. Pat. Nos. 3,578,264 and 3,741,285. KUETHE teaches selected discontinuities positioned both above and below the level of the adjacent flow-control surface which in effect squeeze the boundary layer between adjacent protrudences to eject a vortex into the boundary layer and cause mixing.
It is unfortunate that the prior art vortex generators either operate relatively ineffectively or cause an excess in drag since devices are needed which enable the turning of air flows about sharp cambers. Normally, conventional attempts to reduce the wake and hence the drag of a body moving with respect to a fluid medium by mere curved and tapered body contours in the rear result in pressure recovery regions that are limited by flow separation occurring in the regions of adverse pressure gradients. The flow separation in such instances normally occurs downstream of a body's maximum cross-section.
For example, road vehicles, which are designed for low drag must include very gentle rear deck curvatures to maintain attached flow over a substantial region behind the maximum cross-section. Except in the instance of specially constructed high-speed race cars, full recovery cannot be achieved in the length available on roadable vehicles. A Porsche 924, which is a well streamlined vehicle still has a coefficient of drag (C.sub.D) of approximately 0.35. If all the wake drag caused by flow separation could be eliminated, the C.sub.D could approach 0.25 and result in a considerable fuel savings or speed capability. For comparison, a utilitarian car, such as a Volkswagon Rabbit, (where there is very little attached flow past the maximum cross-section) has a C.sub.D of about 0.45. Elimination of the wake drag caused by flow separation could in theory reduce the C.sub.D to about, 0.30 with no other changes to the vehicle. Although there have been countless attempts to reduce the drag of road vehicles by maintaining attached flow, considerations such as structural weight and aesthetics restrict the length of road based vehicles to such an extent that at some point the body contours must be so abrupt that adverse pressure gradients occur. These produce flow separations which generate a relatively large separation wake. The wake is aerodynamically disadvantageous causing drag and increased fuel consumption especially at higher cruising speeds. This heretofore unavoidable wake is the reason visibly streamlined road cars have heretofore exhibited only small drag advantages over properly designed vehicles with bluff or abrupt tails.
Some vehicles have used wing-like airfoils as turning vanes to assist in directing external flow to reduce the adverse pressure gradients. However, the mass flow of air moved by these add-on devices never comes close to filling the large wake behind a body proportioned like a normal road vehicle. Therefore, the effect is helpful but minor.
There have been suggestions that the application of powered suction at discrete locations near the rear of a vehicle might provide a method for achieving wake reduction. However, wind tunnel studies show that the power required by such suction systems rivals the amount needed to power the original standard vehicle and therefore does not accomplish the overall result of reducing the total energy requirement of the vehicle. If a method could be found to reduce the energy required to move a poor slenderness ratio body through the air by providing means to maintain attached flow in the areas of adverse pressure gradients, the reduced energy requirement would translate directly into reduced fuel consumption.
As reported in NACA Research Memorandum, A8F21 entitled "An Experimental Investigation of a Large Scale of Several Configurations of a NACA Submerged Air Intake", published 19 Oct., 1948, devices are known for generating strong vorticity by passing flow over the sharp corners of the intersections between channel walls and a reference body surface adjacent an inlet ramp surface. Typically, NACA inlets have a ramp surface of generally V-shape which extends to an inlet duct lip thereabove and is bounded on the opposite sides by reflexed sidewalls extending upwardly to the body surface at approximately 90.degree.. However, since all NACA inlets are intended to be inlets for rectangular cross-section intake ducts, their sidewalls must become parallel to the connecting duct sidewalls, hence the reflex and inlet duct lip must be carefully designed. The inlet lip forms the beginning of a fourth wall for the duct at the location of flow entry into the vehicle. The planform of a NACA inlet is always defined by the chosen ramp angle, that is, the included angle between the flat ramp floor and the body's external body surface. The length of the NACA inlet is always a direct result of that choice of angle. This grossly reduces the design choices available, especially when it is desired to reduce the aerodynamic drag of a body having a poor slenderness ratio.
The NACA inlets were designed to be located in regions of positive pressure since their purpose is to take air on board a vehicle, either to provide combustion air to an engine, flow to a heat exchanger or simply provide ventilation. NACA inlets are known to perform poorly in regions of great negative static pressure. NACA inlets are never used for the purpose of influencing external flow conditions on a basic body, other than to take in air and have never been used for the purpose of achieving aerodynamic drag reduction for a body passing through a flowing medium.
Therefore, even though various types of devices are known which produce vortices, none are effective in assuring attached flow in an adverse pressure gradient region with low drag.